JP2015119310A - Surface acoustic wave filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a SAW filter including a structure capable of increasing an attenuation amount in the vicinity of a passband, specifically an attenuation amount on a higher frequency side than the passband, without deteriorating characteristics in the passband.SOLUTION: In a SAW filter 21, a plurality of IDTs 23-25 are arranged along a surface wave propagation direction on a multilayer substrate 22 comprising a multilayer structure of a piezoelectric film/a low acoustic velocity film/a high acoustic velocity film/a support substrate; at least one one-port type SAW resonator 28 is connected in series to the output side of the SAW filter; and an anti-resonance frequency fof the SAW resonator 28 is located on a higher frequency side than a passband of the SAW filter.

Description

  The present invention provides an elastic surface in which a plurality of interdigital transducers (hereinafter referred to as IDTs) are arranged along a surface wave propagation direction on a laminated substrate having a laminated structure of a piezoelectric film / low acoustic velocity film / high acoustic velocity film / support substrate. The present invention relates to a wave filter (hereinafter referred to as a SAW filter), and more particularly, to a SAW filter having a structure capable of increasing the attenuation in the vicinity of a pass band.

  The SAW filter is small and has a steep filter characteristic. Therefore, SAW filters having various structures have been proposed and used.

  FIG. 1 is a schematic plan view showing a three-electrode SAW filter as an example of a conventional surface acoustic wave filter. The SAW filter 1 has a structure in which three IDTs 3 to 5 are arranged on a rectangular piezoelectric substrate 2. Reference numerals 6 and 7 denote reflectors. In the SAW filter 1, one comb-teeth electrode of IDTs 3 and 5 is commonly connected to be an input end, and one comb-teeth electrode of IDT 4 is an output end. The other comb electrodes of IDTs 3 to 5 are connected to the ground potential.

  FIG. 2 is a schematic plan view showing another example of a conventional SAW filter. The SAW filter 8 has a structure in which two IDTs 9 and 10 are arranged on the upper surface of the piezoelectric substrate 2 along the surface wave propagation direction. Reflectors 6 and 7 are arranged on both sides of the IDTs 9 and 10, respectively. In the two-electrode SAW filter 8, one comb electrode of the IDT 9 is used as an input end, and one comb electrode of the IDT 10 is used as an output end. The other comb electrodes of the IDTs 9 and 10 are connected to the ground potential.

  FIG. 3 is a schematic plan view showing still another example of a conventional SAW filter. The SAW filter 11 has a structure in which seven IDTs 12 to 18 are arranged on the upper surface of a rectangular piezoelectric substrate 2 along the surface wave propagation direction, and is called a multi-electrode SAW filter. One comb electrode of IDTs 12, 14, 16, and 18 is commonly connected and used as an input end, and one comb electrode of IDTs 13, 15, and 17 is commonly connected and used as an output end. Further, the other comb electrodes of the IDTs 12 to 18 are each connected to the ground potential.

  In such a multi-electrode SAW filter 11, a large number of IDTs 12 to 18 are arranged, so that insertion loss can be reduced.

  As described above, SAW filters having various structures have been proposed in order to reduce insertion loss. However, when the insertion loss is reduced, the amount of attenuation in the vicinity of the pass band of the SAW filter is not so large. In particular, in the SAW filter using the surface wave resonator as described above, the amount of attenuation in the vicinity of the pass band is small. There was a problem of being small.

  In order to increase the attenuation in the vicinity of the pass band, it is considered that the number of stages of the SAW filter is increased and multiple stages are connected. However, when the number of stages of the SAW filter is increased, the insertion loss increases in proportion to the number of stages.

  Therefore, the appearance of a SAW filter that can increase the attenuation near the passband without increasing the insertion loss is desired. In particular, in mobile communication such as a cellular phone, the frequency interval between the transmission side and the reception side is narrow, and it is necessary to sufficiently secure the attenuation near the passband. However, it has been difficult for conventional SAW filters to satisfy such requirements.

  An object of the present invention is to provide a SAW filter having a structure capable of increasing the attenuation in the vicinity of the pass band, in particular, the attenuation on the high frequency side of the pass band without impairing the filter characteristics of the pass band.

  The invention according to claim 1 is a SAW filter in which a plurality of IDTs are arranged along a surface wave propagation direction on a laminated substrate having a laminated structure of piezoelectric film / low acoustic velocity film / high acoustic velocity membrane / support substrate. At least one 1-port surface wave resonator (hereinafter referred to as a SAW resonator) having at least one IDT is connected in series to at least one of the input / output sides of the SAW filter, and the 1-port The SAW filter is characterized in that the anti-resonance frequency of the type SAW resonator is set higher than the pass band of the SAW filter.

  As a SAW filter in which a plurality of IDTs are arranged along the surface wave propagation direction on a laminated substrate having a laminated structure of the above piezoelectric film / low acoustic velocity film / high acoustic velocity membrane / support substrate, a two-electrode type as described above is used. Alternatively, a multi-electrode SAW filter is also included in addition to the three-electrode SAW resonator filter. In the present invention, in the various SAW filters as described above, the at least one 1-port SAW resonator is connected in series.

According to a second aspect of the present invention, the piezoelectric film is made of LiTaO ₃ , the resonance frequency of the one-port SAW resonator is f ₀ (MHz), the number of electrode fingers is N, and the crossing width is A (μm). When there is one 1-port SAW resonator, the following equation (1)

を満たし、前記１ポート型ＳＡＷ共振子が複数個の場合には、個々の共振子のｆ₀ ／（Ｎ×Ａ）の合計が０．６以下となるように、前記１ポート型ＳＡＷ共振子のＩＤＴが構成されている。

And the one-port SAW resonator is such that the total of f ₀ / (N × A) of the individual resonators is 0.6 or less. IDTs are configured.

According to the first aspect of the present invention, at least one 1-port SAW resonator is connected in series to at least one of the input / output sides of the SAW filter. This one-port SAW resonator is configured such that its anti-resonance frequency f _a is located on the higher frequency side than the pass band of the SAW filter. Therefore, in the passband characteristics of the entire SAW filter, the anti-resonance point of the one-port SAW resonator is located on the high frequency side of the passband, so that the attenuation on the high side of the passband is increased.

In the invention according to claim 2, as will be apparent from the embodiments described later, in the SAW filter using the piezoelectric film made of LiTaO ₃ , the one-port SAW resonator is configured so as to satisfy the above-described formula (1). Therefore, it is possible to effectively improve the attenuation on the high frequency side of the pass band without increasing the insertion loss so much.

  In addition, Formula (1) in the invention of the said Claim 2 is theoretically confirmed by this inventor so that it may become clear from the below-mentioned embodiment.

  According to the first aspect of the present invention, at least one 1-port SAW resonator is connected in series to at least one of the input / output sides of the SAW filter, and the anti-resonance frequency of the 1-port SAW resonator is Since the 1-port SAW resonator is designed so that is positioned on the higher frequency side than the pass band of the SAW filter, the attenuation on the higher frequency side than the pass band can be improved by about 10 to 20 dB.

  Therefore, the amount of attenuation in the attenuation region is increased as compared with the passband, so that the steepness of the filter characteristics is enhanced. Therefore, it is possible to provide a SAW filter suitable for use in applications having a narrow frequency interval, such as the frequency on the transmission / reception side of the mobile phone.

According to the second aspect of the present invention, when LiTaO ₃ is used as the piezoelectric film material, the insertion loss is increased by designing the one-port SAW resonator so as to satisfy the above-described formula (1). Instead, as in the first aspect of the present invention, it is possible to effectively increase the attenuation outside the pass band, particularly the attenuation in the higher frequency range than the pass band.

FIG. 6 is a schematic plan view showing a conventional three-electrode SAW filter. FIG. 6 is a schematic plan view showing a conventional two-electrode SAW filter. The schematic top view which shows the conventional multi-electrode type | mold SAW filter. The schematic plan view for demonstrating the SAW filter which concerns on 1st Embodiment. FIG. 5A is a cross-sectional view of a principal part showing the SAW filter according to the first embodiment. FIG. 5B and FIG. 5C are modifications thereof. The schematic plan view which shows the SAW filter which concerns on 2nd Embodiment. The schematic top view which shows the SAW filter which concerns on 3rd Embodiment. The figure which shows the equivalent circuit of 1 port type SAW resonator. The figure which shows the impedance-frequency characteristic of 1 port type SAW resonator. 36 ° Y-cut -LiTaO ₃ using a substrate, shows the change in insertion loss when varying the value of M. The figure which shows the insertion loss-frequency characteristic of the conventional SAW filter prepared for the comparison. The figure which shows the insertion loss-frequency characteristic of the SAW filter of embodiment which connected four 1 port type SAW resonators.

  Hereinafter, the present invention will be clarified by describing embodiments with reference to the drawings.

  FIG. 4 is a schematic plan view showing the SAW filter according to the first embodiment of the present invention. FIG. 5A is a cross-sectional view of a main part showing the SAW filter according to the first embodiment of the present invention. The SAW filter 21 is configured using a rectangular laminated substrate 22. The laminated substrate 22 has a laminated structure of a piezoelectric film 224 / a low acoustic velocity film 223 / a high acoustic velocity film 222 / a support substrate 221. On the upper surface 22a of the multilayer substrate 22, a plurality of IDTs 23 to 25 are arranged along the surface wave propagation direction on the edge 22b side.

  IDTs 23 to 25 are provided in order to constitute a three-electrode SAW filter. In addition, the reflectors 26 and 27 are arrange | positioned at the both sides of IDT23-25. One comb electrode of the IDTs 23 and 25 is commonly connected as shown and used as an input end. The other comb electrodes of the IDTs 23 and 25 are connected to the ground potential.

  One comb-teeth electrode of the IDT 24 is connected to the ground potential, and the other comb-teeth electrode is connected to the output end OUT via a 1-port surface wave resonator 28 described later.

  The feature of this embodiment is that the 1-port SAW resonator 28 is connected in series between the other comb-tooth electrode of the IDT 24 and the output end, that is, the output side of the 3-electrode SAW filter. is there.

  The SAW resonator 28 includes a pair of comb electrodes 28a and 28b having a plurality of electrode fingers that are interleaved with each other. One comb-teeth electrode 28a is connected to the comb-teeth electrode of the IDT 24, and the other comb-teeth electrode 28b is connected to the output terminal OUT.

The anti-resonance frequency f _a of the SAW resonator 28 is configured so as to be positioned on the high frequency side of the passband of the three-electrode type SAW filter formed by IDT23～25. Therefore, as will be apparent from the description of the operation principle described later, in the filter characteristics of the SAW filter 21, it is possible to increase the attenuation on the high frequency side of the pass band.

  As described above, the SAW filter 21 of the present embodiment includes the multilayer substrate 22 having a multilayer structure of the piezoelectric film 224 / the low acoustic velocity film 223 / the high acoustic velocity film 222 / the support substrate 221, and the IDTs 23 to 25, The reflectors 26 and 27 and the SAW resonator 28 are formed. This will be described in detail below.

  The laminated substrate 22 has a support substrate 221. A high sound velocity film 222 having a relatively high sound velocity is laminated on the support substrate 221. On the high sound velocity film 222, a low sound velocity film 223 having a relatively low sound velocity is laminated. A piezoelectric film 224 is laminated on the low acoustic velocity film 223. IDTs 23 to 25, reflectors 26 and 27, and SAW resonator 28 are laminated on the upper surface of the piezoelectric film 224. Note that IDT 23 or the like may be laminated on the lower surface of the piezoelectric film 224.

  The laminated substrate 22 can be made of an appropriate material as long as it can support a laminated structure including the high sound velocity film 222, the low sound velocity film 223, the piezoelectric film 224, the IDT 23, and the like. Such materials include piezoelectric materials such as sapphire, lithium tantalate, lithium niobate, quartz, alumina, magnesia, silicon nitride, aluminum nitride, silicon carbide, zirconia, cordierite, mullite, steatite, forsterite, etc. Various ceramics, dielectrics such as glass, semiconductors such as silicon and gallium nitride, resin substrates, and the like can be used. In the present embodiment, the support substrate 221 is made of glass.

  The high acoustic velocity film 222 functions so that the surface acoustic wave is confined in a portion where the piezoelectric film 224 and the low acoustic velocity film 223 are laminated, and does not leak into a structure below the high acoustic velocity film 222. In the present embodiment, the high acoustic velocity film 222 is made of aluminum nitride. However, as long as the elastic wave can be confined, aluminum nitride, aluminum oxide, silicon carbide, silicon nitride, silicon oxynitride, DLC film or diamond, a medium mainly composed of the above materials, and a medium mainly composed of a mixture of the above materials. Various high sound speed materials such as can be used. In order to confine the surface acoustic wave in the portion where the piezoelectric film 224 and the low acoustic velocity film 223 are laminated, it is desirable that the film thickness of the high acoustic velocity film 222 is thicker, more than 0.5 times the wavelength λ of the surface acoustic wave, Is preferably 1.5 times or more.

  In the present specification, the high acoustic velocity film refers to a membrane in which the acoustic velocity of the bulk wave in the high acoustic velocity film is higher than that of the surface wave and boundary acoustic wave propagating through the piezoelectric film 224. . The low sound velocity film is a film in which the sound velocity of the bulk wave in the low sound velocity film is lower than the bulk wave propagating through the piezoelectric film 224. In addition, elastic waves of various modes with different sound speeds are excited from an IDT on a certain structure. The elastic waves propagating through the piezoelectric film 224 are used to obtain characteristics of a filter and a resonator. The elastic wave of a specific mode is shown. The bulk wave mode that determines the acoustic velocity of the bulk wave is defined according to the use mode of the elastic wave propagating through the piezoelectric film 224. When the high acoustic velocity film 222 and the low acoustic velocity film 223 are isotropic with respect to the propagation direction of the bulk wave, the following Table 1 is obtained. That is, the high sound velocity and the low sound velocity are determined according to the right-axis bulk wave mode of Table 1 below with respect to the left-axis elastic wave main mode of Table 1 below. The P wave is a longitudinal wave, and the S wave is a transverse wave.

  In Table 1 below, U1 means a P wave as a main component, U2 means an SH wave as a main component, and U3 means an elastic wave whose main component is an SV wave.

上記低音速膜２２３を構成する材料としては圧電膜２２４を伝搬するバルク波よりも低音速のバルク波音速を有する適宜の材料を用いることができる。このような材料としては、酸化ケイ素、ガラス、酸窒化ケイ素、酸化タンタル、また、酸化ケイ素にフッ素や炭素やホウ素を加えた化合物など、前記材料を主成分とした媒質を用いることができる。

As the material constituting the low sound velocity film 223, an appropriate material having a bulk wave sound velocity lower than the bulk wave propagating through the piezoelectric film 224 can be used. As such a material, a medium mainly composed of the above materials such as silicon oxide, glass, silicon oxynitride, tantalum oxide, or a compound obtained by adding fluorine, carbon, or boron to silicon oxide can be used.

  The low sound velocity film 223 and the high sound velocity film 222 are made of an appropriate dielectric material capable of realizing the high sound velocity and the low sound velocity determined as described above.

The piezoelectric film 224, in this embodiment, in LiTaO ₃ namely Euler angles 38.5 ° Y-cut made (0 °, 128.5 °, 0 °) LiTaO 3 , the thickness, IDT23～25 electrode When the wavelength of the surface acoustic wave determined by the period is λ, it is 0.25λ. However, the piezoelectric film 224 may be formed of LiTaO _{3 having} another cut angle, for example, LiTaO _{3 having} a 50 ° Y cut, or may be formed of a piezoelectric single crystal other than LiTaO ₃ .

  In the present invention, since the low acoustic velocity film 223 is disposed between the high acoustic velocity film 222 and the piezoelectric film 224, the acoustic velocity of the elastic wave is lowered. The energy of an elastic wave is concentrated in a medium that is essentially a low sound velocity. Accordingly, the effect of confining the elastic wave energy in the piezoelectric film 224 and the IDT in which the elastic wave is excited can be enhanced. Therefore, compared to the case where the low acoustic velocity film 223 is not provided, according to the present embodiment, loss can be reduced and the Q value can be increased. The high acoustic velocity film 222 functions so as to confine the elastic wave in the portion where the piezoelectric film 224 and the low acoustic velocity film 223 are laminated so as not to leak into the structure below the high acoustic velocity film 222. In other words, in the structure of the present application, the energy of the acoustic wave of a specific mode used for obtaining the characteristics of the filter and the resonator is distributed throughout the piezoelectric film 224 and the low sound velocity film 223, and the low sound velocity film of the high sound velocity film 222 is obtained. It is distributed also to a part of the side and not distributed to the laminated substrate 22. The mechanism for confining the elastic wave by the high acoustic velocity film 222 is the same as that of the Love wave type surface wave which is a non-leakage SH wave.

  In FIG. 5A, a laminated substrate having a laminated structure of piezoelectric film 224 / low acoustic velocity film 223 / high acoustic velocity film 222 / supporting substrate 221 is shown, but as shown in FIG. 225 may be laminated between the support substrate 221 and the high acoustic velocity film 222. Other configurations are the same as those of the first embodiment. Therefore, the description of the first embodiment is incorporated. Therefore, in order from the top, the IDT 23, the piezoelectric film 224, the low acoustic velocity film 223, the high acoustic velocity film 222, the medium layer 225, and the support substrate 221 are laminated in this order.

  As the medium layer 225, any material such as a dielectric, a piezoelectric, a semiconductor, or a metal may be used. Even in this case, the same effect as that of the first embodiment can be obtained. However, when the medium layer 225 is made of metal, the specific band can be reduced. Therefore, it is preferable that the medium layer 225 is made of a metal in an application with a small specific band.

  In addition, as illustrated in FIG. 5C, a medium layer 225 and a medium layer 326 may be stacked between the support substrate 221 and the high acoustic velocity film 222. That is, from the top, the IDT 23, the piezoelectric film 224, the low sound velocity film 223, the high sound velocity film 222, the medium layer 225, the medium layer 226, and the support substrate 221 are stacked in this order. Except for the medium layer 225 and the medium layer 226, the configuration is the same as in the first embodiment.

  The medium layers 225 and 226 may use any material such as a dielectric, a piezoelectric body, a semiconductor, or a metal. Even in that case, the same effect as the surface acoustic wave device of the first embodiment can be obtained.

  In the present embodiment (modification), a laminated structure composed of the piezoelectric film 224, the low acoustic velocity film 223, the high acoustic velocity film 222, and the medium layer 225 and a laminated structure composed of the medium layer 226 and the support substrate 221 are separately manufactured, Both laminated structures are joined. Thereafter, IDT 23 and the like are formed on the piezoelectric film 224. Accordingly, the surface acoustic wave device according to the present embodiment can be obtained without depending on the manufacturing constraints when manufacturing each laminated structure. Therefore, the freedom degree of selection of the material which comprises each layer can be raised.

  Note that any joining method can be used for joining the two laminated structures. As such a bonding structure, various methods such as hydrophilic bonding, activation bonding, atomic diffusion bonding, metal diffusion bonding, anodic bonding, bonding with resin or SOG can be used.

  FIG. 6 is a schematic plan view showing a SAW filter 31 according to the second embodiment of the present invention. Two IDTs 33 and 34 are arranged on an upper surface 32a of a rectangular laminated substrate 32 having a laminated structure of piezoelectric film / low acoustic velocity film / high acoustic velocity membrane / support substrate. One comb-teeth electrode of the IDI 33 is connected to the input end, and the other comb-teeth electrode is connected to the ground potential. One comb-teeth electrode of the IDT 34 is connected to the ground potential, and the other comb-teeth electrode is connected to the output terminal OUT via SAW resonators 37 and 38 described later.

  Reference numerals 35 and 36 denote reflectors. That is, the IDTs 33 and 34 and the reflectors 35 and 36 constitute the same structure as the two-electrode SAW filter shown in FIG.

  In the present embodiment, two 1-port SAW resonators 37 and 38 are connected in series with the SAW filter on the output side of the two-electrode SAW filter. The SAW resonators 37 and 38 have a pair of comb electrodes 37a, 37b, 38a, and 38b, respectively, similarly to the SAW resonator 28 of the first embodiment. However, the comb bar electrode 37b and the comb bar electrode 38a have a common bus bar. As shown in the figure, the comb electrode 37a is connected to the IDT 34, and the comb electrode 38b is connected to the output terminal OUT.

  Also in the second embodiment, the 1-port SAW resonators 37 and 38 are designed so that their anti-resonance frequencies are located on the higher frequency side than the passband of the two-electrode SAW filter. Therefore, as in the case of the first embodiment, it is possible to increase the attenuation on the higher frequency side than the passband.

  FIG. 7 is a schematic plan view showing a SAW filter according to a third embodiment of the present invention. The SAW filter 41 is configured by using a rectangular laminated substrate 42 having a laminated structure of piezoelectric film / low acoustic velocity film / high acoustic velocity membrane / support substrate. On the upper surface 42a of the multilayer substrate 42, a plurality of IDTs 43 to 49 are arranged in the center along the surface wave propagation direction.

  The IDTs 43 to 49 are provided for constituting a multi-electrode SAW filter. One comb electrode of the IDTs 43, 45, 47, and 49 is commonly connected as shown in the figure, and is connected to the input terminal IN via a 1-port SAW resonator 50 described later. The other comb electrodes of the IDTs 43, 45, 47, and 49 are connected to the ground potential.

  Also, one comb electrode of the IDTs 44, 46, and 48 is connected to the ground potential, and the other comb electrode is connected in common and connected to the output terminal OUT via a 1-port SAW resonator 51 described later. ing.

  That is, the SAW filter 41 of the present embodiment is characterized in that one SAW resonator 50, 51 is connected in series on both the input side and the output side of the multi-electrode SAW filter.

Each of the 1-port SAW resonators 50 and 51 is designed such that its anti-resonance frequency f _a is located on the higher frequency side than the passband of the multi-electrode SAW filter formed in the center. Therefore, as in the first and second embodiments, the SAW filter 41 can increase the attenuation on the high frequency side of the passband.

  When connecting a plurality of 1-port SAW resonators as in the second and third embodiments, the SAW filter characteristics are desired by using SAW resonators having different resonance frequencies and impedances. It can be made closer to the characteristics.

  In the first to third embodiments described above, each IDT and reflector may be formed on either the upper surface or the lower surface of the piezoelectric film.

Next, the operation principle of the present invention will be described based on specific experimental results. A one-port SAW resonator is generally represented by an equivalent circuit shown in FIG. Referring to FIG. 8, a 1-port SAW resonator 61 includes an inductance L ₁ , a capacitor C ₁ and a resistor R ₁ connected in series with each other in parallel with the inductance L ₁ , the capacitor C ₁ and the resistor R _1. And a connected capacitor C ₀ . The impedance of the SAW resonator 61 - frequency characteristic is as shown in FIG. 9, the impedance becomes minimum at around the resonance frequency f _0, the impedance becomes the maximum value in the vicinity of the anti-resonator frequency f _a.

Therefore, in the above-described two-electrode or three-electrode SAW filter or multi-electrode SAW filter, when the one-port SAW resonator is connected in series on at least one of the input and output sides, the anti-resonance frequency of the SAW resonator the f _a can be configured to trap the attenuation pole. Thus, by combining a low-loss filter such as SAW resonator filter or a multi-electrode type SAW filter of the two-electrode type or three-electrode type described above, to increase the attenuation in the vicinity of the antiresonance frequency f _a of the SAW resonator be able to.

In this case, setting the antiresonance frequency f _a of the SAW resonator in the high frequency side of the passband of the SAW filter, and if brought into position the resonance frequency f ₀ in the pass band of the SAW filter, the insertion loss in the pass band The amount of attenuation on the higher frequency side than the passband can be increased without deteriorating so much.

  However, when the impedance value at the resonance frequency of the 1-port SAW resonator is high, the impedance matching range becomes narrow, and the insertion loss of the SAW filter increases. Therefore, as shown in the following experimental example, it is desirable to design an IDT of a 1-port SAW resonator to be added according to the configuration of the SAW filter.

Now, assuming that the resonance frequency of a 1-port SAW resonator is f ₀ (MHz), the number of electrode fingers is N (pair), and the crossing width is A (μm), M = f ₀ / (N × A) is The value is proportional to the reciprocal of the interelectrode capacitance of the SAW resonator, and the proportionality constant is determined by the substrate material.

When a plurality of 1-port SAW resonators are connected, f ₀ / (N × A) is obtained for each 1-port SAW resonator, and the sum is the value of M. As the value of M increases, the impedance matching range becomes narrower and insertion loss increases. The amount of increase in insertion loss due to impedance mismatch is practically limited to 0.5 dB, and the amount of increase in insertion loss must be kept below this level. In consideration of the temperature characteristics of the piezoelectric film, the specific band is required to be 1% or more.

FIG. 10 is a diagram showing a result of calculating a ratio band in which the deterioration of the insertion loss is within 0.5 dB when the value of M is changed when 50 ° Y cut-LiTaO ₃ is used as the piezoelectric film. It is. As is clear from FIG. 10, in order to secure a specific bandwidth of 1% or more, the value of M needs to be 0.6 or less.

  As described above, in order to avoid an increase in insertion loss, it is desirable to design the electrode structure of the 1-port SAW resonator so that the M value is not more than a certain value depending on the piezoelectric film material.

  Next, an experimental example in which insertion loss-frequency characteristics are specifically measured for a modification of the SAW filter of the third embodiment shown in FIG. 7 will be described. In the SAW filter 41 of the third embodiment shown in FIG. 7, one SAW resonator 50 and 51 is connected to the input / output side, but insertion when the SAW resonators 50 and 51 are not connected is inserted. The loss-frequency characteristics are shown in FIG.

  Further, a SAW filter in which four SAW resonators 50 shown in FIG. 7 were connected in series was prepared on the output side of the SAW filter whose characteristics shown in FIG. 11 were measured, and the insertion loss-frequency characteristics were measured in the same manner. The results are shown in FIG.

  In FIGS. 11 and 12, a solid line A is a characteristic curve showing an enlarged main part of the passband, and the magnitude of the insertion loss is represented by the scale shown on the right side of FIGS. Has been.

  As is clear from the comparison between FIG. 11 and FIG. 12, in the structure of the embodiment in which four 1-port SAW resonators are connected, the attenuation on the high band side of the passband is improved by about 10 to 20 dB, It can be seen that the insertion loss in the passband has only increased by 0.3 dB.

21 ... SAW filter 22 ... laminated substrate (having a laminated structure of piezoelectric film / low acoustic velocity film / high acoustic velocity membrane / support substrate)
221 ... support substrate 222 ... high sound velocity film 223 ... low sound velocity film 224 ... piezoelectric film 225,226 ... medium layers 23-25 ... IDT
26, 27 ... reflector 28 ... 1-port SAW resonator 31 ... SAW filter 32 ... laminated substrate 33, 34 ... IDT
35, 36 ... reflectors 37, 38 ... 1-port SAW resonator 41 ... SAW filter 42 ... laminated substrate 43-49 ... IDT
50, 51 ... 1-port SAW resonator 61 ... 1-port SAW resonator

Claims (2)

In a surface acoustic wave filter in which a plurality of interdigital transducers are arranged along a surface wave propagation direction on a laminated substrate having a laminated structure of a piezoelectric film / low acoustic velocity film / high acoustic velocity film / support substrate,
At least one one-port surface wave resonator having at least one interdigital transducer is connected in series to at least one of the input and output sides of the surface acoustic wave filter, and the one-port surface wave A surface acoustic wave filter, wherein an antiresonance frequency of the resonator is set to a higher frequency side than a pass band of the surface acoustic wave filter. The laminated substrate is made of a LiTaO ₃ substrate,
When the resonance frequency of the one-port surface acoustic wave resonator is f ₀ (MHz), the number of electrode fingers is N, and the crossing width is A (μm),
When there is one 1-port surface wave resonator,

The filling,
When a plurality of the 1-port surface wave resonators are connected in series, the 1-port surface is adjusted so that the total of f ₀ / (N × A) of the individual resonators is 0.6 or less. The surface acoustic wave filter according to claim 1, wherein an interdigital transducer of a wave resonator is configured.

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